1. Technical Field
The present invention relates generally to short arc discharge lamps and, more particularly, to a short arc discharge lamp in which a cathode electrode is provided with a tip part containing thorium oxide.
2. Description of the Related Art
Generally, conventional short arc discharge lamps filled with mercury come near to a point light source and a distance between front ends of a pair of electrodes that are disposed opposite to each other is comparatively short. Therefore, such a short arc discharge lamp filled with mercury is combined with an optical system and thus mainly used as a light source of exposure equipment that has high light collection efficiency.
Short arc discharge lamps filled with xenon are used as visible ray sources for projectors or the like, and they are currently widely used as light sources for digital cinema.
In such short arc discharge lamps, a cathode electrode may contain an emitter material such as thorium oxide so that electron emission characteristics can be improved.
However, if the cathode electrode is configured such that the entirety thereof contains thorium oxide as an emitter material, the consumption of thorium oxide is increased. This is not preferable in terms of reduction of material usage. Furthermore, because thorium is a radioactive material, there are many regulations with regard to handling it. Therefore, it is substantially difficult to realize a cathode electrode wherein the entire body contains thorium oxide as emitter material.
Given this, as a method of manufacturing a cathode electrode that contains thorium oxide as emitter material, a technique in which a body part of the cathode electrode is made of tungsten and a tip part made of thoriated tungsten containing thorium oxide is solid-phase bonded to a front end of the body part was introduced. A representative example of a cathode electrode produced by the technique was proposed in Japanese Patent Laid-open Publication No. 2012-015007 (Patent document 1).
However, in this conventional technique of manufacturing a cathode electrode having a solid-phase bonding structure, after the tip part made of thoriated tungsten has been solid-phase bonded to the body part that is made of tungsten, when it is machined into a predetermined shape by cutting using a lathe or the like, the cathode electrode may be broken at a junction interface between the body part and the tip part by impact generated during the cutting process.
As such, the conventional technique is problematic in that the bonding strength of the cathode electrode on the junction interface is comparatively low, whereby mechanical fracture may occur from the junction interface.